In an article by R. C. DeVries and W. L. Roth entitled "Critical Evaluation of the Literature Data on Beta Alumina and Related Phases: I, Phase Equilibria and Characterization of Beta Alumina Phases" which appeared in the Journal of the American Ceramic Society of July 1969, Vol. 52, No. 7, pp. 364-369, the authors discuss data in the literature on the system Na.sub.2 O-Al.sub.2 O.sub.3 particularly the sodium aluminate Na.sub.2 OxAl.sub.2 O.sub.3, where x is of the order of 5-8, which has become known as .beta.-Al.sub.2 O.sub.3. .beta.-Al.sub.2 O.sub.3, and a crystallographic variant of it, .beta."-Al.sub.2 O.sub.3, are used as separators in the type of Na-S battery shown and described in U.S. pat. No. 3,404,035 to Kumer et al., which issued Oct. 1, 1968. In such Na-S battery, the anodic reactant is separated from the cathodic reactant by a solid which serves as a barrier to mass liquid transfer, but is selectively conductive with respect to cations of the anodic reactant and substantially impermeable to ions that are formed in the cathodic reactant. Thus, the separator is composed of material that will admit of the transfer of the ions of the anodic reactant through the separator and into the cathodic reactant during the discharge reaction and admit of their selective return when a reverse electric current is passed through the cell during recharging. Moreover, the cathodic reactant, together with the separator, provides a sufficient barrier to free electron flow in the internal portion of the electrical circuit to permit a difference of potential to develop at the respective electrodes when the cell is in operation.